I. Field of the Invention
This invention relates generally to a capacitive transducer apparatus, and more particularly to an accelerometer circuit incorporating a capacitive transducer having a more linear output by reducing the effective parasitic loading capacitance and reducing electromagnetic interference (EMI).
II. Discussion of the Prior Art
Capacitive transducers have numerous uses, such as in a vehicle performance analyzer for detecting changes in G-forces and producing a digital count value proportional to such changes, as described in U.S. Pat. No. 4,694,687 to Bonin et al. incorporated herein by reference. The performance analyzer as taught by Bonin et al. teaches a transducer device comprising first and second pairs of G-force responsive, serially-connected variable capacitors. The transducer device produces output electrical signals proportional to the G-forces experienced by the vehicle in which the performance analyzer is mounted.
There are several problems to be overcome when using presently known capacitive transducers as sensors in a vehicle performance analyzer. Namely, sensors incorporating capacitive transducers have nonlinear characteristics which adversely effect the accuracy of the performance analyzer. This nonlinear characteristic is caused in part by parasitic loading capacitances which are inherent in capacitive transducers in general, and specifically capacitive accelerometers. Such loading capacitance is due to capacitive effects between the output of the transducer devices and other electrical components to which they are coupled, as well as device packaging. A further problem of using capacitive transducers is that their performance is adversely affected by EMI interference generated by adjacent devices, such as electrical components or the engine of a vehicle. While prior efforts have individually attempted to reduce parasitic capacitances or EMI interference, prior art capacitive accelerometers fail to simultaneously reduce both problems.
Referring to FIG. 1, an electrical representation of a prior art capacitive transducer is shown with a grounded shield, and parasitic capacitance represented as C.sub.L which exists between the transducer output and ground. The parasitic capacitance C.sub.L exists between the movable vane or plate and the external environment. Prior art accelerometers typically have a parasitic capacitance C.sub.L equaling 2 pF. Absent any loading parasitic capacitance on the connection between the movable vane and subsequent circuitry, a perfectly linear accelerometer would be obtained.
Since removing parasitic capacitances entirely is impossible, efforts to reduce the effect of the parasitic loading capacitance on the accuracy and linearization of the capacitive accelerometer would be appreciated by one of ordinary skill in the art. Further, an accelerometer including a capacitive transducer which reduces the EMI, which can degrade the accuracy of the sensor, using a practical design would be appreciated as well.